The present invention concerns the field of radiological imaging to visualize an organ or part of an organ, generally of the human body.
Radiography is conventionally carried out with sensitive films exposed by X-rays after they cross the organs to be studied. Radiologists have been trained in the interpretation of such images. The new imaging technologies-solid state detector and digital acquisition systemxe2x80x94must be adapted to common practices and must provide an equivalent perception of the pertinent information that radiologists are accustomed to examining. In particular, one of the requirements that digital systems must satisfy consists of reducing the extent of the gray level dynamics in order to simulate a conventional film as faithfully as possible. For this purpose, the digital image is displayed on a screen that the radiologist adjusts interactively in order to identify all the clinical signs by perceiving the relations between the different image components. But the precise perception of density information by means of the image displayed on the screen is limited by the dynamics of the screen. Now, one must pass automatically from the high contrast of the original image, for example, in the order of 30 to 50, to the low contrast offered by a video screen.
Physicians commonly apply configuration techniques to patients with a view to limiting the dynamics of acquired images, for example, by compression of thick regions and/or by addition of absorbent substances in order to compensate for thin zones. In the case of X-ray mammography, the breast is compressed to the smallest and most constant possible thickness. In the field of cardiology, contour filters are used to avoid problems associated with weakly absorbent zones of the chest, such as the lungs. These filters consist of plates of shape complementing that of the heart, made of materials of given X-ray absorption coefficients.
However, these techniques prove insufficient for digital image acquisition and processing and cumbersome to use, if not uncomfortable for the patient.
The present invention is intended to remedy the above-mentioned problems.
The embodiments of the present invention compensate for variations of thickness, in particular at the limit between high-density regions and low-density regions of the organ X-rayed, in an adaptive manner, depending on characteristics of the image visualized, chosen by a user.
The present invention is intended to offer an image on which tissues presenting absorptions different from one another are naturally and exploitably represented.
The method of compensation of thickness of an organ is designed for an X-ray apparatus of the type comprising an X-ray source and a means of detection of the X-ray beam after it has crossed the organ, the means of detection being capable of converting the X-ray beam into a digital electronic signal. From a digitized image, an image of the radiological thicknesses of organs crossed by the X-ray beam is calculated, the image of radiological thicknesses is filtered by a low-pass filter in order to obtain a low-frequency image, the low-frequency image is subtracted from the image of the radiological thicknesses in order to obtain a contrast image, the low-frequency image is processed by a prerecorded table taking into account a contrast "khgr" chosen by a user in order to obtain an image with reduced dynamics, and the image with reduced dynamics and the contrast image are added to obtain a compensated thickness image. The pixels of level lower or higher than a predetermined threshold are brought back at least to the value of the threshold, while preserving the differences and the real relations between the anatomical structures.
The thickness of an organ measured by X-rays is called radiological thickness, in other words, taking into account the absorption of the materials crossed. For example, 1 cm of bone has the same radiological thickness as 10 cm of water.
The thickness image can be obtained by means of Lambert""s law: I=Ioexe2x88x92xcexct with I the number of photons received at a given point of the means of detection, Io the number of photons which would be received at a given point of the means of detection, if the organ was not present in the field of view, xcexc the coefficient of linear attenuation of the X-rays by the material crossed, and t the thickness of material crossed, from which one deduces:
In Ioxe2x88x921n I=xcexct, the product xcexct corresponding to the gray level for a pixel of a radiological thickness image.
Calculation of the image with reduced dynamics advantageously takes into account a range of gray levels of width and center indicated by a user.
In an embodiment of the invention, the low-frequency image processing for obtaining the image with reduced dynamics is carried out as a function of the width and center of the passband.
In an embodiment of the invention, the low-frequency image processing for obtaining the image with reduced dynamics is carried out by means of a digital table or an analytical law.
In an embodiment of the invention, the low-frequency image processing follows a monotone law.
In an embodiment of the invention, the low-frequency image processing follows a linear law of slope xcex1.
The slope xcex1 advantageously evolves in a manner inversely proportional to the contrast "khgr".
In an embodiment of the invention, the low-frequency image is stored in a memory and, on a change of contrast, the low-frequency image is read in the memory and the necessary processings and calculations are carried out.
In another embodiment of the invention, on a change of contrast, the image of radiological thicknesses is filtered by a low-pass filter in order to obtain a low-frequency image and the necessary processings and calculations are carried out.
The present invention also concerns a system of compensation of thickness of an organ in an X-ray apparatus. The X-ray apparatus is of the type comprising an X-ray source and a means of detection of the X-ray beam after it has crossed the organ, the means of detection being capable of converting the X-ray beam into a digital electronic signal. The system includes a means of calculating, from a digitized image, an image of the radiological thicknesses of organs crossed by the X-ray beam, a low-pass filter for obtaining radiological thicknesses from the image, a low-frequency image, a means for subtracting the low-frequency image from the radiological thicknesses in order to obtain a contrast image, a means for processing the low-frequency image according to a prerecorded table taking into account a contrast "khgr" chosen by a user in order to obtain an image with reduced dynamics and a means for adding the image with reduced dynamics and the contrast image in order to obtain a compensated thickness image, the pixels of level lower or higher than a predetermined threshold having been brought back at least to the value of the threshold, while preserving the differences and the real relations between the anatomical structures.
In an embodiment of the invention, the low-pass filter is calculated to eliminate the pixels corresponding to an organ loaded with contrast medium in the compensation image and thus maintain them in the compensated image.
Thus, the invention offers an image processing supplying an image equivalent to that which would be obtained by using an absorbent liquid on the edges of the organ over a part of their height and makes it possible to simulate a physical phenomenon, which provides a better understanding of the optimal adjustment of the parameters that has to be made. This image processing confers a natural and pleasing appearance to the different tissues of the organ studied. Each of those tissues is seen at the same time and on the same image with, on the one hand, a natural appearance and, on the other, a precision and quality enabling the user to derive important information from it, as if the user were interested only in one particular tissue. In the field of mammography, the glandular zones and the adipose zones are observed simultaneously with the possibility of deriving information from the same image on both of the zones.
The method is adaptable to existing X-ray machines and can be applied to the radiography of any organ at all.
Whatever the extent and centering of the range of gray levels chosen by the user for visualization, a choice made to favor certain types of organs or tissues in the organ studied, the image is processed in a manner adapted to that choice.